by Catalina Camia, USA TODAY

by Catalina Camia, USA TODAY

Democratic Sen. Tim Johnson of South Dakota has reportedly decided not to seek re-election in 2014, which will spark a wide-open battle for the seat in a state President Obama lost last year.

The Associated Press reported Johnson's decision, citing unnamed Democratic officials. Reuters was the first Monday to report the retirement, saying it came from unnamed sources "close to the matter and Capitol Hill staffers" in its report.

Johnson, chairman of the Senate Banking Committee, is set to make a formal announcement about his political future Tuesday at the University of South Dakota, his alma mater.

An open seat in South Dakota would be coveted by both parties, especially Republicans who need a six seats to win the majority. Johnson would be the fifth Democrat to not seek re-election in 2014. Two Republicans have already announced their retirements.

Johnson, 66, was already being targeted for defeat by Republicans. Former governor Mike Rounds announced his intention to run in November, shortly after Obama lost South Dakota by 18 percentage points.

Democrats have a 55-45 voting edge in the Senate. Obama's party has 21 Senate seats to defend in 2014, including in seven states that voted for GOP presidential nominee Mitt Romney last November. Republicans will defend 14 seats.

The other states carried by Romney where Democrats hold Senate seats are West Virginia, Louisiana, Alaska, Montana, Arkansas and North Carolina. Sen. Jay Rockefeller of West Virginia was the first Democrat to announce his retirement. In the other GOP-leaning states, the Democratic senators there have either formally launched their re-election bids or have started raising campaign money, another sign they want to run again.

Johnson, first elected to the Senate in 1996, easily won a third term in 2008. That race was significant because it came two years after Johnson suffered a near-fatal brain hemorrhage. He sometimes slurs his speech and needs a motorized scooter to get around Capitol Hill.

When Rounds announced his Senate campaign, Johnson said he intended to put together "a winning campaign."

South Dakota Democratic Party Chairman Ben Nesselhuf has said Johnson's son, Brendan, is "looking at" the race. Brendan Johnson has been U.S. attorney in South Dakota since 2009. Former congresswoman Stephanie Herseth Sandlin has also been mentioned as a possible Democratic candidate.

On the Republican side, Rep. Kristi Noem has not ruled out a race. It would be the first time since 1978 that there has been an open Senate seat in South Dakota.

As chairman of the banking panel, Johnson oversees the implementation of the sweeping Dodd-Frank law's rules on Wall Street.